I think both sides have some valid points.
First off, 99% of the "anti-freeze" claims are NOT about the PG, it's about the DEG found in one sample by the FDA. Some people confuse the PG as the anti-freeze referred to, but that was not what the FDA was warning about.
While the comment "there is no safe level of diacetyl" sounds ominous, please bear in mind that the anti-tobacco zealots use that very same argument to keep smokers from switching to smokeless tobaccos - which aren't "safe" but are 98-99% safer than smoking. They say "this is not a safe alternative to smoking" while ignoring the benefits of the reduced harm. Nomad is correct that your risk is directly related to the concentration. E-cigarettes have never been intended to be risk-free or safe. They are a reduced harm product, which means that they still carry some risk - as do most things we are exposed to.
Think of it this way - seatbelts & airbags aren't 100% safe, either. There are groups who argue that they are MORE dangerous in an accident, because of deaths attributed to them. But, they are by-and-large attributed to saving more lives than taking them. Even with trace amounts of possible toxins, e-cigarettes have the potential to save many more lives than they could possibly risk.
PG has also not been tested long-term for the way we use it. So, it does seem a little alarmist to worry so much about possible trace elements of one chemical, when PG is still an unknown and nicotine use itself has known risks. If you are worried about it, avoid flavors with diacetyl or are unlabeled.
However, there is the argument that there is no need to add chemicals which are known to have higher risk, when they can so easily be avoided. And that the antis will probably latch onto diacetyl in the same manner as they do the DEG found and that would be a public relations nightmare. It seems foolish to give them any more ammunition. On the flip side, calling out vendors on forums (which the antis do read) only hands them the gun.
I personally believe vendors should list ingredients, including diacetyl, so consumers who are concerned can make an informed decision on their personal comfort level. It's something I've told vendors would be key to their future survival, as people would start voting their trust with their dollars. But the antis won't care that only some vendors use it - they'll point to the industry as a whole, like they did with the diet and erectile drugs some vendors put in e-cigs. So calling out vendors doesn't exactly help the overall cause, either.
So, while I agree that the levels of diacetyl are probably too low for concern - especially in relation to the relative risk of smoking - I also believe it's not the best choice for vendors to continue to use it if there are other options available. However, while discussing and informing vapers about diacetyl is one thing, calling out vendors and causing a panic only gives the antis a handy little list and more unproven reasons to ban e-cigarettes.
IMO, the responsible thing for vendors to do is either not add diacetyl (or any other known toxins which could easily be avoided) or label all liquids for transparency and the responsible thing for vapers to do is to not buy into the anti rhetoric that something needs to be 100% safe in order to improve public health. E-cigarettes are not a cure for nicotine addiction. They are a reduced harm smoking alternative and they are a CHOICE.
So, make your personal choice, but don't start a panic about vendors who use diacetyl or don't label. Simply vote with your dollar and the message will be sent without playing right into the antis hands.